its valves, causes the shell to open and close in a very 

 rapid manner, and it might be thought that at each 

 sudden clapping of the two valves, the Avater between 

 them would be forced out ventrally and that the animal 

 in consequence would move with the hinge line foremost. 

 The free or ventral border of the valves is, however, 

 directed forwards in swimming, and the animal seems to 

 take a series of bites at the water. 



As will be subsequently described, the valves in both 

 species are not mirror images of each other. Pecten 

 maximus has the right valve very much more convex, 

 while the left is quite flat. In P. opercularis the two 

 valves are much more alike but the right is slightly less 

 convex than the left. Pecten maximus lies on the convex 

 valve or right side, and the flat side is, therefore, superior, 

 and is generally covered with barnacles, serpula, 

 zoophytes, &c. P. opercularis also shows by the attached 

 animals being found always on the same side, that it lies 

 on the right valve. If a specimen is turned over on to 

 the other side, it will make efforts to turn back, and 

 usually regains its normal attitude in a few minutes. 

 The two diagrams in text (fig. 1) show that while the two 

 species both lie on the right valve, in one case the more 

 convex side is downwards, and in the other, upwards. 



If the undisturbed animal is watched as it opens the 

 valves (which it does very slowly), the tentacles (Plate 

 II., fig. 1, Tn.) will first be seen gradually protruded, then 

 the eyes will become obvious, and lastly, when the valves 

 are some distance apart, the two free edges of the mantle 

 (which previously lay against the mantle lobes proper) 

 move outwards until they stand almost at right-angles to 

 the plane of the valves, so as to form one curtain or 

 '' velum " (fig. 1, V.) hanging from the upper valve and 

 one projecting up to meet this from the lower valve. 



